Systems and methods of capturing transient elastic vibrations in bodies using arrays of transducers for increased signal to noise ratio and source directionality

ABSTRACT

Provided herein are systems and methods for real time processing of signals from an array of transducers for detecting transient elastic waves originating from unknown locations in a body, which may propagate in a dispersive fashion. The systems and methods allow real time combination and analysis of signals, including decisions regarding storage as new data is received. The methods described herein include designing arrays of detectors and methods for processing signals in real time given the constraints of the body under test determining whether to store the set of information while a new set of information is received for processing within a real time environment. The methods described herein include methods which result in the determination or small time shifts which place all signals into a coherent time base which are then combined achieving a composite waveform that possesses an increased signal-to-noise ratio over any single element.

PRIORITY STATEMENT

This application is a divisional application under 35 USC 120 of U.S.application Ser. No. 15/797,596, filed Oct. 30, 2017 and entitled“Systems and Methods of Capturing Transient Elastic Vibrations in BodiesUsing Arrays of Transducers for Increased Signal to Noise Ratio andSource Directionality,” which claims the priority of provisionalapplication No. 62/480,193, filed Mar. 31, 2017 and entitled “Methodsand Systems for Sensor Spacing in Modal Acoustic Emission Testing.” Thecontents of each of the foregoing applications are fully incorporatedherein for all purposes.

FIELD OF THE TECHNOLOGY

The present disclosure relates to detecting transient elastic waves inbodies with arrays of transducers, detectors, transceivers, orreceivers.

SUMMARY OF THE DESCRIPTION

Provided herein are systems and methods for real time processing ofsignals from an array of transducers for detecting transient elasticwaves originating from unknown locations in a body, which may propagatein a dispersive fashion. The systems and methods allow real timecombination and analysis of signals, including decisions regardingstorage as new data is received. The methods described herein providefor designing the array of detectors and methods for processing thesignals in real time given the constraints of the body under testdetermining whether to store the set of information while a new set ofinformation is received for processing within a real time environmentensuring an increase in sensitivity of the system. The methods describedherein include methods which result in the determination or small timeshifts which place all signals into a coherent time base which are thencombined achieving a composite waveform that possesses an increasedsignal-to-noise ratio over any single element.

In one aspect, the disclosure describes a method including determiningin real time whether to store in a computer memory a first set ofsamples from a plurality of signals from a multi-element transducerarray that is coupled to a body of material under test within a realtime processing environment. The first set of samples represents a firsttime range and ends with a first boundary set of samples that are lateralso processed along with a second set of samples representing theplurality of signals for a second time range possessing a necessaryoverlap at the end of the first time range, thereby creating anoverlapping plurality of processed samples including samples that areprocessed with the first set of samples and processed with the secondset of samples. The overlapping plurality of processed samples issufficient to capture in the multi-element transducer array aslowest-measured wave of interest in the body under test. Theslowest-measured wave of interest is both sensed by a first portion ofthe multi-element transducer array during the first time range andsensed by a second portion of the multi-element transducer array duringthe second time range. The determining in real time whether to store thefirst set of samples further comprises calculating a plurality ofrespective delay times for each of the plurality of signals with respectto a predetermined reference signal of the plurality of signals, suchthat a time-shift by the plurality of respective delay times modifiesthe plurality of signals into a time-base-coherence with thepredetermined reference signal. The determining in real time furthercomprises combining at least one of the time-shifted plurality ofsignals with the reference signal of the plurality of signals, therebycreating a real time combined signal from the plurality of signals. Thedetermining in real time further comprises evaluating the real timecombined signal over the first set of samples for values of the realtime combined signal that exceed a predetermined threshold. Thedetermining in real time further comprises indicating an instruction tostore the first set of samples in the computer memory if, within thefirst set of samples, the real time combined signal crosses apredetermined threshold in the real time combined signal.

In another aspect, the disclosure further describes a system including abody of material under test that is adapted to detect propagatingtransient elastic vibrations possessing a slowest moving wave componentas transmitted through the body of material from a source of thattransient elastic wave deformation information to the multi-elementtransducer array. The system further includes a maximum pitch of themulti-element transducer array coupled to the body pre-determined bycomputational and memory limits of the hardware and its ability todetermine coherency of the signals from the multi-element transducerwhile maintaining the required positive overlap so as to ensureincreased sensitivity. The system further includes a receiver circuitfor processing a plurality of signals received from multi-elementtransducer array. The receiver circuit further comprises a circuit fordetermining a time delay between a first reference signal of theplurality of signals and the other signal(s). The receiver circuitfurther comprises a combination circuit for creating a combined signalbased on the time shifted signals. The receiver circuit furthercomprises a threshold-detecting circuit for detecting in real timevalues of an output of the combination circuit above a threshold attimes when none of the plurality of the received signals may have avalue over the threshold.

Other embodiments and features of the present disclosure will beapparent from the accompanying drawings and from the detaileddescription which follows.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an exploded view of an exemplary transducer array asdescribed herein, including associated electronics and channels for eachof the transducer elements.

FIG. 2 shows an exemplary graph describing an overlap of samples for usein determining in real time whether to store a first set of samples.

FIGS. 3A-3C illustrate the angular dependence on phase velocity oftransient elastic vibrations during transmission along a hoop direction,an axial direction and 45 degrees to the axial direction of an exemplarytest composite vessel.

FIG. 4 show the effects of source orientation of a vibration source withrespect to a transducer array.

FIG. 5 shows additional effects of source orientation of a vibrationsource with respect to a transducer array.

FIG. 6 shows details of an exemplary transient elastic wave as measuredby individual elements from a source at a parallel angle of incidence.

FIG. 7 shows signals detected from a parallelly orientedpencil-lead-break source to an array of transducers, along with adigitally summed and shifted signal as described herein.

FIG. 8 shows signals detected from a perpendicularly orientedpencil-lead-break source to an array of transducers, along with adigitally summed and shifted signal as described herein.

FIG. 9 shows timing differential calculated for various angles ofincidence for summation of signals in an exemplary linear array of fourelements.

FIG. 10 shows increases in flexure mode amplitude for alternativesummation possibilities for a body under test as compared to the methodsdescribed for digital real time summation herein.

The embodiments are illustrated by way of example and not limitation inthe Figures of the accompanying drawings in which like referencesindicate similar elements.

DETAILED DESCRIPTION

The following patent description and drawings are illustrative and arenot to be construed as limiting. Numerous specific details are describedto provide a thorough understanding. However, in certain instances,well-known or conventional details are not described in order to avoidobscuring the description. References to one or an embodiment in thepresent disclosure are not necessarily references to the sameembodiment; and, such references mean at least one. Reference in thisspecification to “one embodiment” or “an embodiment” or the like meansthat a particular feature, structure, or characteristic described inconnection with the embodiment is included in at least one embodiment ofthe disclosure. The appearances of the phrase “in one embodiment” or thelike in various places in the specification are not necessarily allreferring to the same embodiment, nor are separate or alternativeembodiments mutually exclusive of other embodiments. Moreover, variousfeatures are described that may be exhibited by some embodiments and notby others.

FIG. 1 shows an exemplary transducer array as described further hereinwith four transducer elements and four separate data channels fortransmitting data for processing and/or storage. The transducer array isarranged between interface portions 10 and an array housing 6, which mayhave a removable lid 1 and may be pressurized via ports 4. The housing 6contains electronics, such as shown on circuit board 7. Interfaceportions 10 may be covered or arranged by an optional barrier 3 (e.g., awater tight barrier) that is held by frame 2 and fasteners 5 intended tocreate a compliant fluid filled bladder. In certain measurementscenarios, (e.g., when needing to couple to a rough or highly irregularsurface, or when automating sensor placement) it is not possible tocouple traditional sensors with a stiff interface between the part undertest and the piezoelectric element; in these scenarios, the compliantfluid filled bladder enables coupling to the part under test. By fillingthe compliant bladder with a fluid (where only dilatational wavepropagation is supported) the fidelity of the measured waveforms may beacceptably preserved with adequate coupling.

The array housing 6 shows an equal number of ports for relaying datafrom the circuit board 7 for further processing. In another embodimentseveral or all of the channels of data may also be multiplexed togetheronto a single port or channel for the purpose of transmission.

However, regardless of how the signals and data are transmitted fromtransducer through the electronics, to complete the real time processingdescribed herein, the samples of data received must be processed by themethods without loss of data. As described further herein, operatingthese methods in real time and without a loss of data requires anoverlap of sample processing and effectively an overlap of samples inthe system.

FIG. 2 shows an exemplary graph describing an overlap of samples for usein determining in real time whether to store a first set of samples. Thedisclosed methods utilize arrays of particular configurations to processin real time signals from the arrays in manners that increase signal tonoise ratios (SNR) for the received signals beyond those of any of thesingle channels. The graph shows the positive sample overlap that isrequired to detect events by an array without the possibility of missingan event that could cross the thresholds described and claimed herein,thus ensuring real time performance and collection of data.

A single exemplary event is illustrated as it would be detected by eachof the transducers of the array, and the single event is captured by thesufficient positive sample overlap. This positive sample overlapillustrates the outcome of a set of engineering decisions forimplementing the systems and methods described herein. The illustratedwave in the single exemplary event represents the slowest-moving wavecomponent in the frequency band of interest for the body under testdetermined from the dispersion relations (or bulk wave propagationvelocities, as appropriate), wave modes of interest, and transducerarray geometry. Each of these aspects of this real time determinationwill be defined in various embodiments further herein.

The disclosed methods allow for determinations to be made in real timebased on the materials under test, and the specific transducer arraycharacteristics and geometries. These decisions must be made withsufficient information about events that might be missed due to theevent arriving to parts of the array in one set of samples and arrivingin other parts of the array in another set of samples. For example, areal time decision may be made within the real time environmentdescribed herein about storing, transmitting, and/or further processingfor each set of N samples such that data and/or events are not lost ormissed by these decision-making processes. These decisions allow for anadvanced real time triggering decision that facilitates an increase inthe signal to noise ratio (SNR) of the array as compared to the SNR forindividual elements of the transducer array. These decisions includeprocesses for detecting a set of time differences for coherentlycombining the data, combining the data into a composite signal, andanalyzing the signal before making a decision on storing the informationand beginning processing on the next set of data received.

The overlap of samples shown in the figure represents an amount of timein which the samples may be processed by the analysis processes herein(e.g., including decisions made regarding storage) on a first set ofsamples while a second set of samples are received. The overlap in timeof the first set of samples and the second set of samples ensures thatfor a given sample rate, the slowest moving wave component must bedetected in the same set of samples so as to enforce the increase in SNRof the array.

Therefore, the boundary set of samples is processed with both the firstset of samples and the second set of samples. As used herein, the termsboundary set of samples and sample overlap may be used interchangeablyto describe this set of samples that is processed with both the firstset of samples and the second set of samples, with no gap of samplesbeing processed by the method in real time. The real time processingrequirement may be expressed as a positive overlap in processing time,specifically for a positive overlap that there is more processing timeavailable than processing time required to process (e.g., analyze,combine, store) a set of samples, including related boundary samplesets, while another set of samples is received for processing.

Timing requirements for real time processing requires a positive sampleoverlap as shown in the figure. This positive overlap creates theability to capture an exemplary event within the array and process theentirety of the event within the time represented by one set of samples.In this way, the array of transducers may properly capture the transientelastic waves in the body under test coming from an event located at anyangle relative to the array. The largest overlap required for real timeprocessing occurs events registering the largest timing differentialbetween the events arriving at each of the transducers, as measured bythe slowest moving wave of the transient elastic waves coming from theevent within a frequency band of interest. Therefore, the positivesample overlap, or the processing time overlap, that is required forreal time processing may be calculated from the separation of thetransducers receiving the transient elastic waves in the body undertest, sample rate, and the speed of the slowest moving wave of anexpected event to be detected.

As described further herein, the combination of signals by the methodsherein requires that at least two signals (and often more, such as four,nine, sixteen, etc. signals) detected from the event must be combined inorder to maintain the described gains in the effective signal to noiseratios of the transducers. These gains may be necessary to detectcertain events above the noise level or to increase the spacing ofsensors while still maintaining adequate coverage of the structure, asdescribed further herein. Therefore, the methods described herein mayinclude systems with specific characteristics that are used to ensurereal time processing may be achieved without losing data.

For example, as shown in FIG. 2, an initial N samples representing 13seconds of monitored time (t=0-13 seconds) has designated as a boundaryset of samples as its final 3 seconds of time (from 10-13 seconds)before a second set of N samples begins (e.g., at 13 seconds on thegraph). Thus, in the exemplary embodiment shown in the figure, in orderto process the initial N samples in real time, the samples representingfrom 0 seconds to 13 seconds must be stored in physical memory and theirtime coherency determined while the second N samples are received fortheir own processing (digitization, physical memory storage, and timecoherency determination). To ensure the real time increases by the arrayin the SNR of the combined signal, the computational power of thehardware must be such that the time coherency of the sets of N samplesand their respective boundary sets is determined, and a triggeringdecision made (either passing data on to a PC for storage if thethreshold was exceeded, or clearing the physical memory space if atrigger did not occur) prior to the available memory overflowing.

Shown in the figure are a boundary set of samples representing timesrepresenting 10 seconds to 13 seconds. This boundary set of samples isprocessed both with the initial set of N samples as well as beingprocessed with the second set of N samples representing times from 13seconds to 23 seconds. This process is thereafter repeated, for examplewith samples from 20-23 seconds forming a second boundary set of samplesbetween the second set of N samples and a third set of N samplesrepresenting data from 23-33 seconds. This second boundary set ofsamples would be processed both with the second set of N samples as wellas the third set of N samples.

In this manner, as shown in FIG. 2, despite the event arriving on thetop channel after 13 seconds and after the first N samples (and firstboundary set of samples) have completed, the event may still be capturedby the analysis of the second set of N samples that also includes thefirst set of boundary samples. In some instances, the event would bemissed by the analysis of the first set of N samples and boundarysamples alone. For example, in some instances the event may be missed ifthe first N samples and boundary samples did not contain enough of theevent such that summation of the signals as described herein did notbring the combined signal over threshold.

As another example, consider transient elastic vibrations propagating ina plate-type structure having calculated dispersion relations. Further,consider a source with dominant out-of-plane source orientation thatpreferentially excites the flexure mode. From the dispersion relationsof the plate/material property combination, it is determined that theslowest moving component of the flexure mode for the lowest frequency ofinterest in the test is 0.8 mm/μs. The final information necessary todetermine the necessary time overlap is the array geometry andinter-element pitch. For this example, consider a four (4) element 1Darray with inter-element pitch of 15.9 mm. From this information, it isdetermined that in the limiting case (parallel source orientationrelative to a 1D array) the maximum distance for the wave to traversethrough the array will be from element four and element one (or viceversa). From the velocity of the slowest moving wave component (0.8mm/μs), it is determined that a minimum time overlap of 59.625 μs isrequired to ensure no gap in data acquisition to enforce the real timeincrease in SNR of the array.

The timing and sample sizes discussed herein for this figure areexemplary and may be markedly different in various applications based onthe specifics of arrays of transducers monitoring bodies under test,wave velocities, and frequencies of interest. For example, while certainarrangement and processing constraints may be illustrated by aninter-element pitch of 15.9 mm or by a sample overlap representing aboundary time frame of 3 seconds, other applications may have differentpitches or much shorter boundary times, as calculated as describedherein based on transducer dimensions and/or characteristics of themeasured elastic waves.

Specific discussions for choosing the sampling frequency for embodimentsare not included in this disclosure because the techniques are welldiscussed in digital signal processing works. In general, sampling ratesmust be sufficient such that the highest frequency component of interestsatisfies the Nyquist frequency criterion and is not eliminated by ananti-aliasing filter.

FIGS. 3A-3C illustrate the angular dependence on phase velocity oftransient elastic vibrations during transmission along a hoop direction,an axial direction and 45 degrees to the axial direction of an exemplarytest composite vessel. FIG. 3A shows a set of dispersion relations ofelastic waves travelling along a hoop direction of the compositepressure vessel. FIG. 3B shows a set of dispersion relations of elasticwaves travelling along an axial direction of the composite vessel. FIG.3C shows a set of dispersion relations of elastic waves travelling at 45degrees to the axial and hoop directions of the composite pressurevessel. Other bodies under test may have different bulk ultrasonic orbulk seismic wave velocities and arrays of transducers may use thesedifferent wave velocities as described further herein to determineproper array geometries and numbers of transducers as well as todetermine a location of an event. From FIGS. 3A-3C, it is clear thatmode velocity can be highly dependent upon propagation direction of thatmode in the body under test. A dispersion relation may include differentwave modes (e.g., extensional wave modes, flexural wave modes) of theelastic waves through the body under test. These various characteristicsof the transient elastic waves may be used to design the transducerarray and to modify the methods herein (e.g., maximum number of lags toconsider in a time shifting algorithm, or number of computationsnecessary for time coherency determination) for real-time analysis andstorage decision making.

Techniques in active phased array ultrasonic testing (PAUT) may useinverse time delays that are imposed on the detected signals for eachelement according to the focal law in order to time shift all detectedwaveforms into a zero time register and the resulting time-shiftedcoherent waveforms are then summed to produce a composite Amplitude Scan(or A-Scan). Conventional phased array sensing provides for aconsiderable level of increase in the sensitivity (and SNR) of receivedsignals for PAUT. Utilization of the increase in detection sensitivityfrom the summation of signals from the phased array in PAUT, where bulkultrasonic waves propagate from a known focal point, is astraightforward endeavor as the ultrasonic waves are non-dispersive(i.e., velocity is constant for all frequencies), and the focal law maybe utilized to set the appropriate time shifts for each element so as toshift each elements waveform back into a zero register. In passivewaveform measurement situations (e.g., Modal Acoustic Emission (MAE)testing, seismology, etc.) where sources are broadband, wave propagationcan be both dispersive, and sources originate at unknown locationsrelative to the array a closed form set of time delays is not applicablein time shifting the waveforms from multiple elements into coherence.However, using the methods described herein to analyze the signals inreal time, SNR gains analogous to those achieved in PAUT applicationsmay be achieved for transient elastic waves measured in situations whereclosed form time reversal laws are not applicable (i.e., broadbandsources, dispersive wave propagation, and/or unknown sourceorigination).

The tested composite pressure vessels allow for precise geometry andconfiguration measurements to be made in order to establish the realtime ability of the methods described herein for processing in realtime. These tests on composite pressure vessels confirmed that themethods correctly determined the proper timing differentials in realtime despite the dispersive, multi-mode, attenuating, and othercharacteristics of the acoustic waves transmitted therein. For otherbodies under test with broadband sources, dispersive wave propagation,or source origination from unknown locations, such that they that do notlend themselves to conventional time reversal techniques, these bodiesmay use the techniques herein for real time analysis of signals based ontheir ability to determine the proper timing differentials withoutfitting a closed form set of timing differentials to each eventdetected.

The composite materials in a composite test vessel provide aviscoelastic media in which the transient elastic waves are measured.The polymer matrix in a composite material may be comprised of aviscoelastic resin such that attenuation increases with frequency. Anexemplary composite material can attenuate higher frequencies many timesmore than lower frequencies. Some analyses of these bodies may requirereview of frequencies in the body, and as used herein, the term“frequency under test” or “frequency of interest” means a frequency forwhich analyses are required or used for the specific analyses beingperformed (e.g., to properly identify a damage mechanism originatingfrom within the microstructure, or to identify a location of an event inthe body).

In addition, each of these transient elastic waves may have a separatespeed of travel through the medium, caused by dispersion of thetransient elastic waves representing an event, and creating additionalneed for maintaining a positive overlap. Therefore, for design purposesof the array of transducers, the frequencies of interest will beanalyzed for a “slowest moving wave component” of the frequency ofinterest which is used to designate herein the portion of the transientelastic waves with the slowest velocity in the subject material. Asdescribed further herein, the slowest moving wave may be used todetermine several of the required parameters of the arrays oftransducers and required computational power of the time-shiftinghardware using the methods herein.

In addition, in the presence of material anisotropy the slowestvelocities may be different in different directions. Therefore, thearray configurations must consider the characteristics of incident wavesand the need for capturing the entirety of an event possibly in boundarysample sets processed with both surrounding sets of samples.

As further described herein, the acceptable overlap of processing timesand samples may be controlled during operation of the systems andmethods herein to adjust operation and detection of events in the bodyunder test. For example, while processing times may fluctuate betweenevents, an additional process monitoring the desired and actual overlapsavailable and used for each detected event may be employed to monitorthe methods herein.

FIGS. 4 and 5 show the effects of alternate orientations of a transientvibration source with respect to a transducer array. The transientvibration source (e.g., an event, material facture, etc.) inducestransient elastic waves in the body under test. This source will producea broad spectrum of vibration frequencies that may each have atransmission speed based on the elastic medium and geometry. The methodsherein adapt an array of transducers measuring the body under test suchthat the slowest moving wave propagating from the source will bedetected by each of the elements at least within the overlap time orrelated sample overlap available during processing.

The methods described herein provide for processing in real time sets ofdata from signals with overlap between the sets of data consisting of amaximum time difference between the event arriving and completing ateach of the transducer elements in the array. In one embodiment, thearray configuration and processing is adapted such that events from anylocation in the body under test may be captured by the array with apositive overlap regardless of angle of propagation, capturing theslowest moving waves and allowing combination of an entire event even iffrom a source angle that maximizes detection delay between the elementsin the array (e.g., aligned parallel with the axis of a 1D array).

In some embodiments, more overlap is available due to computationalprocessing efficiencies than is required by the physical arrayconfiguration, the slowest moving transient elastic wave component, andthe temporal extent of relevant events. In alternate embodiments, thearray configuration and processing are adapted to adjust the overlapbetween angles from zero overlap to a positive overlap.

As shown in FIG. 4, a source of transient elastic waves emanating froman angle perpendicular to a linear (1D) array (i.e., at an angle of 90degrees) will deliver the transient elastic waves to the transducerarray with a minimum of delay between the elements. For example, if thesource or event is far enough away from the transducer array, a planarassumption may be made for the waves and the arrival time differencebetween transducer array elements will be near zero.

Alternatively, as shown in FIG. 5, a source of transient elastic wavesemanating from an angle that is in line with a linear (1D) transducerarray (i.e., at zero degrees, parallel to an axis of the array) willhave a maximum delay time between transducer signals for the array. Forexample, the source waves incident on the first element will need to bedelay-matched with each successive element in the array by an integernumber of delays based on the position of the transducer producing therespective signal. As described further herein, other angles may createdelays which require different delay times to bring the signals intotime coherency. These different angles may create different SNRincreases in the combined signal based on the combining efficiencies andthe described methods' ability to correlate and cancel the noise asdescribed herein.

FIG. 6 shows details of an exemplary transient elastic vibration wave asmeasured by individual elements from a source at an angle of zerodegrees (i.e., wave propagation in a parallel direction to the array).An incident wave with particular speed based upon the dispersionrelations, and coming from a direction in line (i.e., parallel) with anaxis of the transducer array. This direction maximizes the timedifferential between signals received by each transducer, while othermasking distortions will also be present between the received transientelastic waves (e.g., due to dispersion, attenuation, electronic noise).As described further herein, time differentials between the signalsreceived may be determined in real time to combine signals intocoherence with a designated element in the array (e.g., element 2,element 3), thus increasing the effective SNR in real time and allowingenhanced processing steps (e.g., normalizing, storing decision in realtime). By bringing each of the signals into coherence with a centralelement of the array, such as element 2 or element 3 in the linear (1D)4 element linear array, the processing requirements may be reduced bycreating smaller adjustments to each signal. The following figuresinclude measured signals from linear (1D) arrays of four elements,including the summed signals as produced in real time ensuring themethods described herein.

FIG. 7 shows signals detected from a pencil lead break source at 0relative degrees to an array of transducers, along with a digitallysummed and shifted signal as described herein. In this embodiment, thesource location is the same as shown in the prior figure, emphasizingthe time differential between the signals. As shown the induced event(i.e., a pencil lead break) creates waves that are first detected onChannel 4, then Channels 3-1 in sequential reverse order. As describedfurther herein, the combination of the signals (e.g., time shifted andsummed signal), shown in the fifth trace at the bottom, shows a combinedsignal with a greater amplitude than each of the constituent signals. Inanother embodiment, the summation of the signals may be scaled,normalized, or otherwise modified to create a better combined signal.

As shown in the combined signal, the signal to noise ratio for theindividual signals is reduced via the processes herein (i.e.,correlation, time differential calculations, combination), and this SNRgain for the combined signal is detailed in later figures. The SNR gainsare dependent on the angle of the incidence of the waves from the sourceon the array, the body under test (e.g., its dispersive and othercharacteristics), and the frequency content of the transient elasticwaves from the source, but are shown achievable and robust using themethods described herein.

In one embodiment, the timing differential calculations for combinationare created by determining the maximum of the cross correlation productbetween the signal from a predetermined reference element (e.g., element2) and the signals from each of the other individual elements. However,there are other time shifting algorithms and/or calculations that may beadapted for use within the present real time environment that may createacceptably accurate timing differentials to complete the processesdescribed herein. Therefore, one or more of the several existingalgorithms and/or calculations may be adapted to create the combinedsignals used by the methods and systems described herein.

Cross-correlation is a mathematical construct which provides a measureof the similarity of two time-series as a function of the lag in samplepoints of one signal relative to the other. The below equation providesthe mathematical definition of the m^(th) cross-correlation coefficientfor two signals x and y, each of which has length N.

${{\hat{R}}_{xy}(m)} = \begin{matrix}\sum_{n = 0}^{N - m - 1} & {m \geq 0} \\{{\hat{R}}_{yx}\left( {- m} \right)} & {m < 0}\end{matrix}$

By time shifting the two series through a number of lags, the amount ofshift between the two series may be determined by the point at which thetwo signals are most similar or when the waveforms exhibit maximumcoherency (i.e., {circumflex over (R)}_(xy) is maximum). From aconceptual view point, FIGS. 4 and 5 shows how each of the fourindividual elements of the phased array will receive the wave fields atslightly offset times from one another as the stress waves propagatethrough the medium. Through the use of cross-correlation, the waveformscaptured by all four elements in the array can be shifted to appear asif they were all detected simultaneously. As an example of determiningan upper limit on the number of computations that would be required fora given array, the worst case relative to the amount of inter-elementtime difference (i.e., parallel wave incidence) is considered, FIG. 6.From inspection of FIG. 6, it is seen that the wave field will incidentelement 4 first, and element 1 last. Assuming the slowest movingcomponent of the flexure mode (i.e., lowest frequency of interest) formost engineering materials has a velocity of no more than 0.5 mm/μs inmost practical MAE inspection scenarios, an equation to establish thelargest amount of time (or lags) needed to shift waveforms into registermay be written as

$t = {\frac{n \cdot {dx}}{c_{f}} = \frac{{n \cdot 15.9}\mspace{14mu} {mm}}{0.5\mspace{14mu} {{mm}/{\mu s}}}}$

where n equals the number of elements to shift, dx is the inter-elementspacing or pitch, and c_(f) is the slowest moving flexure mode velocity.In this example and orientation, shifting all waveforms to elements 2require n=2 (t=63.6 μs or 318 lags if sampled at 5 MS/s), whereasshifting to elements 1 or 4 would require n=3 (t=95.4 μs or 477 lags ifsampled at 5 MS/s). Therefore, to minimize the number of requiredcomputations and without loss of generality, all waveforms were shiftedto element 2 which will henceforth be referred to as the referenceelement. While the computational requirements of the time shiftingalgorithm may be reduced by selecting an advantageous reference element,the maximum width (or length depending upon orientation) of number ofelements of the array to cover must be considered when determining thesample overlap requirements.

FIG. 8 shows signals detected from a pencil lead break source at 90relative degrees to an array of transducers, along with a digitally timeshifted and summed signal as described herein. In this embodiment, thesource is located at 90 degrees to an axis of the array, thus allowingthe transient elastic waves to reach each of the transducers nearlysimultaneously. However, the methods described herein may still be usedto increase SNR as shown in the combined signal trace.

As in other embodiments, this embodiment may use the same timedifferential calculations and processing techniques as described for usewith any other event emitting transient elastic waves from any sourcelocation. To be clear, a system and methods described herein will notknow, in most embodiments, anything about a source location of thewaves, and thus the same determinations and calculations will benecessary for each set of samples received. In other embodiments, someinformation is known about the source location and adaptations may bemade to the methods and systems herein based on this information.Therefore, the amplifications and triggering decisions for each channelmust account for variations in received waves from different angles.

FIG. 9 shows timing differential calculated for various angles ofincidence for summation of signals in an exemplary linear array of fourelements. As anticipated, for a source with perpendicular (90°)orientation, the time shift between elements is relatively low and issteadily increasing as source orientation nears a parallel (0°) sourcelocation.

Also, the timing differentials shown in the figure are all determinedwith the reference element being an interior element (element 2) of thelinear array. In several embodiments, the timing differentials arecalculated for an interior element of the array, minimizing timingdifferentials and the requirements on processing power between samplesets. Furthermore, the required timing differentials for summing thesignals may be different for a particular element based on the materialor body under test and calculating those differentials in real timeafter receiving signals from the event allows for minute but importantchanges in the timing differentials to be made in order to optimize thereal time detection and SNR-increasing aspects of the systems andmethods herein.

FIG. 10 shows increases in flexure mode peak amplitude for alternativesummation possibilities for a body under test as compared to the methodsdescribed for digital real time summation herein. Each row of the figureprovides results comparing direct analog versus digital time shiftingfollowed by summation from different angles of propagation. The reportedsummation gains that are available via analog summation provide formodest SNR gains over a single channel, however, they do not increaseSNR sensitivity by as much or as consistently as the disclosed digitallyshifted and summed signal. The disclosed combined signals provide theSNR gains reported in the phased array modal acoustic emission (PA-MAE)systems.

Increases in SNR are important for the methods and systems describedherein, but the consistency of SNR increases is also an important factorin system design for the goal of providing full coverage of a body undertest while minimizing the number of arrays that must be placed on thestructure. By increasing the SNR, the number of required transducerarrays can be reduced by increasing the effective dynamic range of thesystem.

Therefore, the real time processing requirements must be met to preparea triggering scheme. the analog summation techniques shown cannotprovide the benefits of the methods and systems herein because they bothprovide a lesser gain in SNR and provide a varying SNR gain depending onthe angle of propagation. Because the systems and methods herein areused with arrays to detect waves at any angle of propagation, variancein the SNR increase can further complicate the process of triggeringevents being detected.

Thus, the methods and systems described herein must be adapted tomeasure the transient elastic waves to provide the proper SNR boostsacross many or all angles of propagation. The methods and systems allowreceiving signals with increased SNR from an unknown source location inthe material under test with dispersive characteristics, while stillallowing the system to operate in real time.

In some embodiments, arrays using the systems and methods herein may beable to create these SNR gains covering many angles of source location,allowing a smaller array to monitor a larger body under test. Forexample, a pressure vessel may be held within a transportation containerand an array may be attached only to a portion that is accessible, yetit will still be able to apply those SNR gains to sensing eventsoriginating in other portions of the pressure vessel. By accounting forthe dispersive characteristics in the material under test, the methodsherein allow an array with designed pitch to process in real time thetransient elastic waves originating from any location on the materialbody under test, including while a portion of the body under test notbeing accessible. For example, these methods and systems may be used tonecessitate a smaller number of sensor arrays and/or arrays that can beused on materials under test without physical access to a portion of thebody of material.

This patent description and drawings are illustrative and are not to beconstrued as limiting. It is clear that many modifications andvariations of this embodiment can be made by one skilled in the artwithout departing from the spirit of the novel art of this disclosure.While specific parameters, device configurations, parameters ofcomponents, and thresholds may have been disclosed, other referencepoints can also be used. These modifications and variations do notdepart from the broader spirit and scope of the present disclosure, andthe examples cited here are illustrative rather than limiting.

What is claimed is:
 1. A method comprising: determining in real timewhether to store in a computer memory a first set of samples from aplurality of signals from a multi-element transducer array that iscoupled to a body of material under test within a real time processingenvironment, wherein: (a) the first set of samples represents a firsttime range and ends with a first boundary set of samples that are lateralso processed along with a second set of samples representing theplurality of signals for a second time range possessing a necessaryoverlap at the end of the first time range, thereby creating anoverlapping plurality of processed samples including samples that areprocessed with the first set of samples and processed with the secondset of samples; (b) wherein the overlapping plurality of processedsamples is sufficient to capture in the multi-element transducer array aslowest-measured wave of interest in the body under test, wherein theslowest-measured wave of interest is both: i. sensed by a first portionof the multi-element transducer array during the first time range; andii. sensed by a second portion of the multi-element transducer arrayduring the second time range; wherein the determining in real timewhether to store the first set of samples further comprises: calculatinga plurality of respective delay times for each of the plurality ofsignals with respect to a predetermined reference signal of theplurality of signals, such that a time-shift by the plurality ofrespective delay times modifies the plurality of signals into atime-base-coherence with the predetermined reference signal; combiningat least one of the time-shifted plurality of signals with the referencesignal of the plurality of signals, thereby creating a real timecombined signal from the plurality of signals; evaluating the real timecombined signal over the first set of samples for values of the realtime combined signal that exceed a predetermined threshold; andindicating an instruction to store the first set of samples in thecomputer memory if, within the first set of samples, the real timecombined signal crosses a predetermined threshold in the real timecombined signal.
 2. The method of claim 1, wherein the determining inreal time whether to store a first set of samples is a first determiningin real time, the method further comprising: second determining in realtime whether to store a second set of samples from the plurality ofsignals such that the second set of samples includes both the firstboundary set of samples and a second set of boundary samples that areboth also processed as part of the second set of samples; and completingthe second determining in real time such that the overlap of processedsample sets is sufficiently maintained between the second set of samplesand a third set of samples that represent times following the secondtime range to capture the slowest-measured wave of interest in the bodyunder test containing the second set of boundary samples.
 3. The methodof claim 2, wherein the sufficiently-maintained overlap of processedsamples allows the slowest-measured wave of interest in the body to beshifted into the time-base-coherence with the predetermined referencesignal within a limited range of delay-shifts; wherein the limited rangeof delay-shifts is a multiple of a longest-delay-unit equal to aninter-element pitch of the multi-element transducer array divided by aslowest wave speed of interest in the body under test.
 4. The method ofclaim 1 wherein the real time combined signal reaches the predeterminedthreshold at a delay-adjusted time, and wherein, at such delay-adjustedtime, none of the plurality of signals individually necessarily includesa value at or above the predetermined threshold.
 5. The method of claim1, wherein the determining in real time further comprises: indicatingthe instruction to store the first set of samples after receiving thefirst boundary set of samples and while receiving a completed set of thesecond set of samples.
 6. The method of claim 1, wherein the firstcombining in real time further comprises: having completed the firstcombining in real time when the second time occurs.
 7. The method ofclaim 1, wherein the first combining in real time further comprises:having completed the first evaluating the real time combined signal overthe period of time while the second time occurs.
 8. The method of claim1, wherein the multi-element transducer array includes a maximum pitchbased on the slowest moving frequency component of interest of thewaveform that can be transmitted, the number of delays between themulti-element transducer required to be considered for combination, andthe ability of the hardware to maintain real time data sampling.
 9. Themethod of claim 1, further comprising: amplifying at least one of theplurality of signals by an amount that creates more noise in the atleast one of the plurality of signals before combining the at least oneof the plurality of signals into the real time combined signal.
 10. Themethod of claim 1, wherein the predetermined reference signal isreceived from a designated reference element of the multi-elementtransducer array.
 11. The method of claim 1, further comprising: furthermodifying the combined at least one of the time-shifted plurality ofsignals and reference signal by normalizing the combined signal bydividing it by a weighted value related to the number of signals used tocompute the combined signal.
 12. The method of claim 1, wherein thecombined signal possesses an increased signal-to-noise ratio (SNR) thatis greater than any individual SNR of any single element.
 13. The methodof claim 12, further comprising: operating the system with a set ofamplifications of signals from the individual elements that results in agreater usable dynamic range of the combined signal than any dynamicrange of any single element.
 14. The method of claim 13, furthercomprising: calculating a number of transducers necessary to monitor thebody under test based both on the greater usable dynamic range of thecombined signal and on the increased SNR of the combined signal,allowing for fewer transducer array placements.
 15. The method of claim1, further comprising: calculating a location of an event that causedthe real time combined signal to cross the predetermined threshold usinginformation about the plurality of respective delay times andinformation about an arrangement of the multi-element transducer array.16. The method of claim 16, further comprising: separating informationabout at least two wave modes from the received plurality of signals;and wherein calculating the location of the event further comprises:using information about a dispersion relation of the body under testwith respect to the multi-element transducer array to range the source;and using information about the at least two wave modes.
 17. The methodof claim 1, further comprising: separating information about at leasttwo wave modes from the received plurality of signals; and whereincalculating the plurality of respective delay times for each of theplurality of signals further comprises for inverse beam forming: usinginformation about a dispersion relation of the body under test withrespect to the multi-element transducer array; and using informationabout the at least two wave modes for source ranging to determine sourcelocation.